1. Field of the Invention
The present disclosure is directed toward an optical component such as, for example, a mirror in a projection objective. Electromagnetic radiation emitted by a light source impinges on an optical surface of the optical component, and consequently, heats the optical component to a maximum temperature Tmax. The optical component is fabricated of a material having a temperature-dependent coefficient of thermal expansion α(T).
Such optical components are of particular interest especially in the field of X-ray lithography. This applies in particular for lithography with soft X-rays, namely the so-called extreme ultraviolet (EUV) lithography in the wavelength region of 10 to 30 nanometers (nm). Mirrors with a high reflectivity in the X-ray region are used as optical components in the area of X-ray lithography. Such mirrors can be operated close to the perpendicular incidence as so-called normal-incidence mirrors or in grazing incidence as grazing-incidence mirrors. Mirrors are designated as grazing-incidence mirrors where rays of a ray pencil impinge upon the mirrors at angles of α>70° relative to the surface normal.
2. Description of the Related Art
X-ray mirrors with a high reflectivity in the X-ray region that are operated as normal-incidence mirrors comprise a substrate material and, based on the same, a multi-layer system, e.g. a Mo/Si multi-layer system or a Mo/Be multi-layer system or a MoRu/Be multi-layer system. With such systems it is possible to achieve reflectivity of over 50% or even of over 60% in the EUV range.
Depending on a wavelength of light to be reflected, it is also possible to use layer systems of other materials.
In addition to X-ray mirrors that are operated as normal-incidence mirrors it is also possible to use mirrors that are operated in grazing incidence, namely so-called grazing-incidence mirrors. Such mirrors also comprise a substrate material. A simpler layer system is applied to the substrate material however. The applied layer can be a ruthenium, palladium or rhodium layer.
For X-ray mirrors used in EUV lithography, and especially in projection objectives, high image-forming qualities are should be achieved.
Since the X-radiation, as explained above, is never reflected completely either under normal incidence or under grazing incidence, energy is introduced into the mirrors, and the mirrors or the respective optical components heat up, that is, increase in temperature. The heating of an optical component causes a thermal expansion that influences image-forming quality of the optical component.
EP 0 955 565 discusses minimization of thermal effects in optical components used in EUV projection systems. For the purpose of suppressing the thermal effects, the mirrors known from EP 0 955 565 comprise a metallic substrate as substrate material. As a result of the favorable thermal conductivity of the metals, the heat introduced into the mirrors is removed efficiently via the rear sides of the metal substrates preferably by a cooling apparatus. The aberrations due to mirror deformations are minimized by this discharge of heat.
A disadvantageous aspect in the solution according to EP 0 955 565 is that the minimization of aberrations introduced by the heat occurs in such away that the heat introduced into the optical component is removed actively by the cooling device. This requires a complex set-up. Moreover, additional components always bear a risk for failure.
A further disadvantage in the use of metals as substrate material is that it is necessary, in order to achieve the smoothest possible surface, to coat the metal substrate with a thin film of an amorphous substance as an intermediate layer. The intermediate layer is polished in order to achieve a sufficiently low roughness. It is only on this layer that the optical layers of the EUV components are applied, e.g., the multiple layer systems for the normal-incidence mirrors or the optical coatings for the grazing-incidence mirrors.